I. Field of the Invention
The present invention relates to error correction coding. More particularly, the present invention relates to a novel and improved technique for performing iterative coding and decoding.
II. Description of the Related Art
"Turbo coding" represents an important advancement in the area of forward error correction (FEC). There are many variants of turbo coding, but most types of turbo coding use multiple encoding steps separated by interleaving steps combined with the use interative decoding. This combination provides previously unavailable performance with respect to noise tolerance in a communications system. That is, turbo coding allows communications at very low E.sub.b /N.sub.0 levels that were previously unattainable using the existing forward error correction techniques.
Many systems use forward error correction techniques, and therefore would benefit from the use of turbo coding. For example, turbo codes could improve the performance of wireless satellite links, where the limited downlink transmit power of the satellite necessitates receiver systems that can operate at low E.sub.B /N.sub.0 levels.
Digital wireless telecommunication systems, such as digital cellular and PCS telephone systems, also use forward error correction. For example, the IS-95 over-the-air interface standard, and its derivatives, such as IS-95B, define a digital wireless communications system that uses convolutional encoding to provide coding gain to increase the capacity of the system. A system and method for processing RF signals substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System" assigned to the assignee of the present invention and incorporated herein by reference ('459 patent).
Because digital wireless communication system like IS-95 are primarily for mobile communications, it is important to have devices that minimize power usage and which are small and light. Typically, this requires development of a semiconductor integrated circuit ("chip") for performing most or all of the necessary processing. While convolutional encoding is relatively complex, the circuits necessary to perform convolutional encoding and decoding can be formed on a single chip along with any other necessary circuitry.
Turbo coding (in particular the decoding operation) is significantly more complex than convolutional encoding. Nonetheless, it would be highly desirable to include turbo coding in digital wireless telecommunication systems, including mobile digital communication systems. Thus, the present invention is directed to reducing the complexity associated with performing turbo coding, and therefore facilitating the use of turbo coding in systems where mobility is desirable, or where power consumption and circuit size are of concern.